1. Field of the Invention
This invention relates to lubricating oil compositions that are particularly useful as functional fluids in systems that require transfer of power by fluid coupling or hydraulic action, and most particularly automatic transmissions for powered vehicles, e.g. automobiles. Automatic transmission fluids must serve a number of functions including lubrication of gears and bearings, transfer of heat, and provision of a fluid for hydraulic control and for transmitting hydraulic power. Because of the various demands placed on a fluid of this nature it is necessary that the fluid be compounded with several types of additives that will impart the desired characteristics. Among these types of additives are included viscosity index improvers, oxidation inhibitors, corrosion inhibitors, dispersants, pour point depressants, antifoaming agents, de-emulsifiers, antiwear agents, seal swellants and friction modifiers. It is obvious that all of these types of additives must be satisfactorily compatible with each other so that the presence of one type will not interfere with the desired function of another type; thus, the compounding of an acceptable automatic transmission fluid is no easy matter.
The present invention is concerned with improvements in the friction performance of a power transmission fluid that is useful in systems that rely on the frictional coupling of two or more otherwise unconnected surfaces that are immersed in or in contact with the fluid, such as in an automatic transmission. The invention involves the use of a friction modifier of a certain type in conjunction with a minor quantity of an overbased detergent salt, preferably an overbased metal hydrocarbon sulfonate, overbased metal alkyl phenate, or overbased metal salt of a sulfurized alkyl phenol, which unexpectedly enhances the friction modifying action of the friction modifier.
In an automatic transmission there are provided a turbine drive unit, a torque converter and one or more friction brakes or clutches which are engaged and disengaged automatically by an intricate hydraulic control unit. A representative clutch assembly comprises plain steel plates that come into contact with steel plates faced on both sides with a friction material such as compressed paper, impregnated with a resin. Because the clutch assembly is in contact with the transmission fluid the frictional properties of the latter have a great effect on the performance of the automatic transmission. It is most desirable that the transmission shift smoothly and that "lock-up" take place within a certain specified period of time when shifting from one transmission speed to another. It is the function of a good friction modifier to ensure that the desired performance will be attained and maintained.
A related application of functional fluids in systems employing frictional coupling is in farm tractors and other modern agricultural machines that use oil-immersed brakes and power takeoff clutches running in a common oil supply with the transmission. Here again the friction characteristics of the fluid are important so that the demands of efficient noiseless braking and power-takeoff clutch capacity can be met.
2. References To The Prior Art
There are teachings in the prior art of the use of metal salts of hydrocarbon sulfonic acids and of alkylated phenols in functional fluids such as lubricants for wet clutch systems, wet braking systems and automatic transmissions.
U.S. Pat. No. 3,410,801 describes a hydraulic fluid suitable for a wet clutch system which comprises a hydrocarbon lubricating oil containing the reaction product of an overbased metal hydrocarbon sulfonate and a fatty acid.
U.S. Pat. No. 3,451,930 teaches a lubricant for farm tractor transmissions comprising a lubricating oil containing a metal salt of a hydrocarbon sulfonic acid, a zinc salt of a dialkyl dithiophosphoric acid and chlorinated paraffin wax.
U.S. Pat. No. 3,259,583 shows a power transmission fluid consisting essentially of a major amount of a mineral lubricating oil, from about 5 to about 15 wt. % of an overbased alkaline earth metal petroleum sulfonate and minor amounts of zinc oleate and N,N'-bis(dialkyl aminophenyl) ether.
U.S. Pat. No. 3,920,562 teaches the use of a detergent amount of a metal salt of a hydrocarbon sulfonic acid in a functional fluid lubricating oil composition that also contains an alkenyl succinimide dispersant, a metal dihydrocarbyl dithiophosphate antioxidant and a friction modifying amount of a hydroxy fatty acid ester of a dihydric or polyhydric alcohol.
U.S. Pat. No. 3,899,432 discloses a functional fluid lubricating oil that is useful both as a hydraulic fluid and as a transmission and differential lubricant and which contains both a neutral metal salt of a hydrocarbyl sulfonic acid and an overbased metal salt of a hydrocarbyl sulfonic acid along with a metal salt of a dihydrocarbyl dithiophosphoric acid, tricresyl phosphate, and a sulfurized mixture of olefins and fatty acid esters.
U.S. Pat. No. 4,010,106 describes a functional fluid particularly useful in automatic transmissions which comprises a lubricating oil containing an alkenyl succinimide dispersant, a metal dihydrocarbyl dithiophosphate antioxidant, an overbased sulfurized alkaline earth metal alkyl phenate detergent-dispersant, a chlorinated olefin as a corrosion inhibitor, and a friction modifier, the latter being a certain type of fatty acid ester, fatty acid amide or fatty alkanol amine. A metal salt of a hydrocarbyl sulfonic acid may be present in conjunction with the sulfurized metal alkyl phenate.
In British Pat. No. 1,452,513 there is taught a lubricant for use in a wet brake system which comprises a lubricating oil, a detergent or a dispersant, a fatty acid, and a fatty acid amide. The detergent can be a metal organo sulfonate or a metal alkyl phenate.
A hydraulic fluid that is not intended for use in automotive transmissions but rather is designed for use in systems that are subject to contamination by sea water, such as in ships, submarines, diving rigs, underwater hydraulic tools, and the like is the subject of U.S. Pat. No. 3,981,813. The fluid comprises a hydrocarbon oil base containing four additive components, one of which is a metal hydrocarbyl sulfonate that serves as a rust and corrosion inhibitor and a sea water emulsifier. The other components are an alkyl or alkenyl succinic acid or anhydride of about 50 to 90 carbon atoms, an alkyl or alkenyl succinic acid or anhydride of about 12 to 34 carbon atoms and an ethoxylated fatty alcohol or an ethoxylated alkyl phenol.